In the process for forming flat glass by the float process a stream of molten glass is delivered onto an elongated pool of molten metal (usually tin or alloys thereof) within a forming chamber. As the layer of glass floats on the molten metal, it is attenuated to form a ribbon of glass of predetermined thickness. As the glass progresses through the forming chamber it is permitted to cool by loss of heat to the molten metal and to the atmosphere within the chamber so that at the exit end of the forming chamber the ribbon may be lifted from the molten metal without deforming.
It is conventional in float forming chambers to provide the roof of the chamber with arrays of electric heating elements so as to control the rate and pattern with which the glass cools. Because the glass tends to lose heat faster near the sides of the forming chamber it is often desirable to provide more heat to the side portions than to the center portions of the forming chamber. Power can be applied to more of the roof heaters overlying the marginal areas of the glass than to those over the central portion to compensate for the faster cooling at the sides, but because of the distance between the roof heaters and the glass such a differential heating arrangement does not direct the heat to the marginal portions as controllably as is desired. Conventionally, heating elements in float forming chambers have been located generally above the elevation of the side access openings so as to not interfere with the attenuating devices inserted through the side seal areas.
In U.S. Pat. No. 4,322,235 float forming chamber heaters are arranged above only the marginal portions of the glass, but are likewise located in the upper portion of the chamber and thus do not focus their heat as specifically onto the marginal portions of the glass as would be desired.
U.S. Pat. No. 3,533,773 shows a low edge heater in a float chamber, but is limited by design to a specific location near the area where the molten glass is delivered into the forming chamber.
Thus, it can be seen that there is a need for means to apply heat to marginal edge portions of the glass in a float chamber from an elevation closely adjacent to the glass surface. Moreover, it would be highly advantageous for such a heating means to be adaptable to being employed at various locations along the side of the float chamber so as to optimize its effectiveness and to adapt to process changes.